\documentclass{article}

\usepackage{pgfextra}
\usepackage{debugPGF}

\begin{document}
\newcommand{\fortest}[1]{#1 is \pgfkeys{/fd/#1}}


\pgfqkeys{/fd/tititi}{.initval=112}

\pgfkeys{/fd/tititi=titi}
after \verb+\pgfkeys{/fd/tititi=titi}+, \fortest{tititi}.

\tracingmacros=1
\tracingassigns=1%

\pgfkeys{/fd/tititi/.reinit} 

\tracingmacros=0
\tracingassigns=0%
after \verb+\pgfkeys{/fd/tititi/.reinit}+, \fortest{tititi}.


%\debug{\keyList}


\pgfkeys{/tutu/.initial=\pgfkeysnovalue}

\pgfkeys{/tutu/.add if non empty={[}{]}}

'\pgfkeys{/tutu}'

\pgfkeys{/tutu=1}

\pgfkeys{/tutu/.add if non empty={[}{]}}

'\pgfkeys{/tutu}'

\pgfkeys{/tutu=}

\pgfkeys{/tutu/.add if non empty={[}{]}}

'\pgfkeys{/tutu}'

\end{document}
